1. Field of the Invention
This invention relates generally to mechanical ventilators and respiratory aids and more particularly to patient effort sensing means whereby the ventilator may be commanded to an appropriate operating mode.
2. Description of the Prior Art
While presently available respiratory systems can furnish operating modes of various parameter bases, it becomes important with very weak patients to sense an almost infinitesimal attempt to breathe. On the other hand, excessive pandering to the respiratory needs of the patient can build up a harmful addiction of the patient upon the ventilator such that the patient will be unable to breathe on his own even after he has physically recovered sufficiently so to do.
Modern respirators, therefore, have been developed in response to patient need, and may thus operate in a number of modes among which are commonly included a control mode. In the control mode, a patient is totally dependent on the ventilator for his respiratory needs. That is to say, the ventilator completely controls all the respiratory functions of the patient including rate of breathing, volume of breathing gas to be inhaled by the patient, and all other rates and pressures permitted to and furnished by the ventilator to the patient.
In another mode, the assist mode, in response to an initial effort by the patient to breathe, the ventilator "assists" the patient by delivering a predetermined breath to the patient. The breath so delivered is in all ways similar to the above-described controlled breath except that the rate of breathing is determined by the patient himself in response to his efforts to breathe. In the pure assist mode, therefore, no breath is furnished to the patient until he makes an effort to breathe on his own. Stated in another way, if the patient does not attempt to breathe on his own, no breath will be furnished to him by the machine. It is thus of the utmost importance that every effort, no matter how feeble, that the patient makes no breathe be sensed. If the effort is not sensed, then no breath can be furnished to the patient as a result of this effort and he remains dependent upon the backup provisions, if any, of the ventilator. Thus his dependence upon the machine would become increased and his addiction thereto exacerbated.
Another mode of lesser interest in the instant invention is denominated the spontaneous mode. In the spontaneous mode the ventilator functions primarily to furnish blended breathing gas to the patient as required by him by maintaining a constant positive airway pressure (CPAP). In this mode, the patient must be strong enough to draw sufficient breathing gas into his body to constitute a complete breath. In the ventilator machine to be described as an exemplar for the purpose of explaining the instant invention, the spontaneous mode will have little, if any, application.
Although some prior art respirators have been satisfactory in their operation to a point, it has been found that they were lacking in sensitivity in some respects. The needs of some patients were thus not met under some conditions. For example, one patient may be able to initiate a stronger breathing impulse than another. If the weaker patient's effort is not also sensed, however, the purpose of the assist mode is not being implemented in his case. It thus becomes important to mechanize as sensitive a system as possible in order to sense every effort, no matter how feeble, on the part of a respirator patient to breathe.
Many different schemes have been employed in attempts to solve the respirator problems presented. Most have either presented new problems or only partially solved the problems presented, or both. Most of these devices have thus met special needs as presented by specific problems and have, therefore, served narrow purposes. These prior art devices, among other disadvantages, have been unreliable and unpredictable in operation under continued use and have been expensive and complicated to manufacture. Some of these prior art devices have been described in the following listed patents that were brought to the attention of the applicant through a novelty search conducted in the United States Patent and Trademark Office:
______________________________________ U.S. Pat. No. Title Patentee ______________________________________ 3,903,876 Respiration Monitor T. R. Harris 3,645,133 Electronic Spirometer P. G. Simeth 3,438,254 Fluid Flow Detector E. B. Seeley 3,368,212 Gas Flow Monitor S. D. Klyce 3,316,902 Monitoring System for H. T. Winchel Respiratory Devices 3,085,431 Flow Measuring Apparatus A. J. Yerman ______________________________________
Upon examination of the above listed patents, the following analyses were made by Applicant and are submitted herewith for the convenience of the Examiner.
With respect to the Respiration Monitor, U.S. Pat. No. 3,903,876, to Harris, it is observed that this device would not be suited for use with a ventilator because it is not intended for use in closed system. The tubes are open to the atmosphere at several points including right at the patient, which construction will not operate satisfactorily with a positive pressure ventilator. The device has three additional disadvantages, among others, when compared with Applicant's invention. First, this device is designed to sense flow irrespective of direction. It is thus possible that an extra breath could be delivered when the patient needs to exhale rather than to inhale. Second, because all directional flow is sensed, there may be less dead time in which the thermistor can recover. And third, it is felt that it would be impossible to achieve the sensitivity that has been achieved with Applicant's invention.
With respect to the Electronic Spirometer, U.S. Pat. No. 3,645,133, to Simeth et al, it is observed that this device places a thermistor in line to measure patient flow with a second thermistor placed separately for temperature compensation. Since the thermistor is located in the main line, the cross-section at that point must be sized for full patient flow. The result is that a relatively large volume of air must move past the thermistor which means poor sensitivity and poor response time in comparison with Applicant's invention.
With respect to the Fluid Flow Detector, U.S. Pat. No. 3,438,254, to E. B. Seeley, it is noted that this device is also a main line sensing system subject to the disadvantages mentioned above. It also appears that this method is more complex and bulky than Applicant's invention.
With respect to the Gas Flow Monitor, U.S. Pat. No. 3,368,212, to S. D. Klyce, it is noted that a pressure differential between patient system and atmosphere caused by patient flow is relied on to cause airflow through a tube to cool a thermistor. Since the device described herein does not use a closed system, it would not be suitable for use with a positive pressure ventilator.
The greatest part of the patent to H. T. Winchel et al for a Monitoring System for Respiratory Devices, U.S. Pat. No. 3,316,902, deals with the circuitry and basic principles of breath rate monitors and patient assist function. The thermistor probe in this device is mounted directly to the body of the patient. There are several problems with this approach. Among them: (1) There is a shock hazard involved whenever an electrical device is mounted directly to a patient. If the thermistor were insulated sufficiently well to prevent all danger, the sensitivity or response time would be adversely affected. (2) At the patient, there is a great deal of humidity, mucus, and miscellaneous foreign matter which could accumulate on the thermistor and thus affect its performance. (3) Anything outside the ventilator which must be handled by therapists, nurses, and other personnel is subject to abuse and breakage. (4) If the thermistor is in contact with the patient, it needs to have provisions for disconnecting it from the ventilator to sterilize it. It must also be able to withstand sterilizing temperatures. It is felt that these disadvantages weigh very heavily against the device of this patent in respect of Applicant's invention.
Another interesting patent brought to Applicant's attention is the Flow Measuring Apparatus, U.S. Pat. No. 3,085,431 to A. J. Yerman et al. One major disadvantage of the device described in its complexity. It is designed for accurate measurement rather than merely to sense an attempted initiation of flow.
It would thus be a great advantage to the art to provide a system of greater sensitivity to patient effort than that presently available.